1. Field of the Invention
The present invention relates to a carbon steel wire material which is made of high carbon steel as a raw material for wire products such as steel cords, bead wires, PC steel wires and spring steel, allows for these wire products to be manufactured efficiently at a high wire drawing rate and has excellent wire drawability.
2. Description of Related Art
To manufacture the above wire products, wire drawing is carried out on a steel wire material as a raw material for the control of size and material (mechanical properties) in most cases. Therefore, the improvement of the wire drawability of a steel wire material is extremely useful for the enhancement of productivity and the like. When wire drawability is improved, many advantages such as the improvement of productivity by an increase in wire drawing rate and a reduction in the number of passes for wire drawing and also the extension of the service life of a die can be enjoyed.
As for wire drawing, researches have been mainly focused on wire breakage resistance at the time of wire drawing. For example, patent document 1 discloses technology for improving wire breakage resistance by optimizing the size of a pearlite block, the amount of proeutectoid cementite, the thickness of cementite and the Cr content of cementite, paying attention to these.
Patent document 2 reveals that the wire drawing limit is improved by controlling the area ratio of upper bainite and the size of bainite contained. Further, patent document 3 discloses technology for improving where breakage resistance and the service life of a die by controlling the total amount of oxygen contained in steel and the composition of a non-viscous inclusion. As for the service life of a die, the descalability of the surface of a steel wire material is also important. If scale remains on the surface of a steel wire material due to poor descalability, it causes the chipping of the die at the time of wire drawing. Therefore, patent document 4 discloses technology for improving mechanical descalability by controlling pores existent in scale.
However, the above prior arts place main emphasis on the improvement of wire breakage resistance under specific wire drawing conditions and rarely pay attention to the improvement of wire drawing rate, the reduction of the number of passes for wire drawing and the extension of the service life of a die from the viewpoint of wire drawability. As previously disclosed, increases in wire drawing rate and the area reduction rate per pass lead to the deterioration of the ductility of wire products and the shortage of the service life of the die. However, the effect of improving wire drawability to such an extent that increases in wire drawing rate and area reduction rate can be achieved at practical levels is not obtained yet from the above prior arts.    Patent document 1 JP-A2004-91912 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)    Patent document 2 JP-A 8-295930    Patent document 3 JP-A 62-130258    Patent document 4 Japanese Patent No. 3544804